Silicon-assisted synthesis of thiocarbonyl derivatives and reactivity of dienophilic thioaldehydes

Treatment of bis(trimethylsily1) sulfide with CoC12-6H20 in the presence of aldehydes affords the corresponding thiocarbonyl analogues which can be trapped to avoid polymerization. The sulfurization reaction also takes place in the presence of TfOSiMe3, in which case, besides thioaldehydes, thioketones may be obtained in satisfactory yields. When thioaldehydes are generated with the CoCl26H20 method the Diek-Alder reaction with cyclohexadiene occur with very high selectivity in favor of the endo isomer, whereas when the TfOSiMe3-based method is employed, the stereochemistry of the cycloadduct can be conveniently selected toward endo or exo by varying the molar ratio of the sulfurating agent